1. Field of the Invention
This invention related generally to work holders for electrical circuit assemblages and more particularly to automated insertion of the pins of various multiple lead electronic devices into corresponding pin holes in a printed circuit board.
2. Background Description
Robots are used for the assembly of electronic equipment such as for mounting multiple lead IC chips and other electronic components on circuit boards. Examples of such devices include ceramic chip carriers, relays, transformers, IC components, resistor network components and other components having pins or fragile leads which may, for example, be on 0.050 inch centers. The components typically include from 24-54 such pins which are about 10.5 mils in diameter which are to be inserted in holes which are about 14-18 mils in diameter.
Present known robots use servo-systems and are programmed to position a robotic arm specific distances in the X-Y axes, i.e. in a plane parallel to the board to accomplish alignment, and, once reaching the proper X-Y coordinates, to then move in the Z axis, i.e. vertically toward and away from the board to accomplish insertion of the leads into holes in the board. The board is semi-flexible and the periphery of the board is supported. As a result, the board has some compliance in the Z axis. Also, the flexibility of the leads provides additional compliance in the Z direction. Further, Bernoulli type and vacuum type holders, or remote center compliant devices attached to the robotic arm, provide compliance while holding the device for insertion. The total X-Y-Z compliance provides beneficial deflection in the assembly system which aids in limiting damage to the devices and the board during insertion attempts. Some of these holders, referred to as end effectors, are also compliant in rotation expressed in terms of .theta..
Limitations of robotic assembly include bent or deformed pins which preclude insertion, mis-positioning of the robotic arm in the X-Y axes, and pin vs. hole tolerance which may be inconsistent.
Some known remote center compliant devices include resilient members of elastomeric material which deform to avoid jamming during insertion of parts. Sensory feedback causes the robotic arm to withdraw and try again if an insertion attempt is unsuccessful. Rapid repetition of these attempts is required to make the automation fast and therefore economically justifiable. However, a hysteresis effect is present which delays reestablishment of the end effector to a null position or initial precision registration point relative to the X, Y, Z, .theta. compliance. This hysteresis effect is caused by deformation of the elastomeric material which relatively slowly restores to the null position. This is because restoring forces in the material grow weaker as the device approaches re-establishment of the initial registration. Thus, when subsequent attempts are being made, initial precision registration of the end effector may not have been fully restored which further complicates the problem of automated pin to hole insertion.
The foregoing illustrates limitations known to exist in devices similar to the present invention. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.